New York: Powerful storms carrying high winds, torrential rain and hail slammed heavily populated parts of the US Northeast on Tuesday, knocking over trees that killed two people, snarling transport and causing widespread power outages.

An 11-year-old girl was killed on Tuesday afternoon when strong winds caused a tree to fall on a parked car in Newburgh, New York, the Daily Freeman newspaper in Kingston reported.

A man who had been mowing his lawn and sought refuge in his truck was killed when a tree fell on the vehicle in Danbury, Connecticut, Danbury Mayor Mark Boughton said, according to the Hartford Courant newspaper.

There were nearly 50 reports of hail in states including Ohio, Pennsylvania, New York and Connecticut, the National Weather Service (NWS) said.

Much of the ferocity of the storms had dissipated by 10:30pm ET (0230 GMT) but flash flood warnings were still in effect for eastern Maryland, Delaware and southern New Jersey, where rain continued to be heavy, said Frank Pereira, a meteorologist with the NWS.

Hail the size of tennis balls smashed car and house windows in Hartford, Pereira said.

Nearly 500,000 customers in New York, New Jersey, Connecticut, Pennsylvania and Virginia were without power by late Tuesday night, fewer power outages than earlier Tuesday, tracking service PowerOutage.us said.

At Grand Central Station in New York, Hudson, Harlem and New Haven Line service was suspended due to the storms, the station's Twitter feed said.

"I got off the subway at about 5:15, and there were hordes of people," said Jackie Berman, who was attempting to get home to Chappaqua, New York, from Grand Central. "It's a mess."

More than 500 flights were cancelled at the three major airports serving the New York on Tuesday, and more than 100 at Boston's Logan International, according to tracking service FlightAware.com.

Rain is expected to continue for the next couple of days over much of the areas that experienced strong winds and rain on Tuesday, but the threat of severe weather has diminished, Pereira said.